United Nations climate chief Christiana Figueres is calling for world leaders to aim for an even lower threshold for rising global average temperatures, even as new figures suggest that the prospects for preventing temperatures from rising beyond a key benchmark grew dimmer in 2010.

According to estimates released this week by the International Energy Agency, global emissions of energy-related carbon dioxide in 2010 were the highest ever measured at 30.6 gigatonnes -- a 5 percent jump over the previous record year of 2008.

The increase follows a decline in global emissions in 2009 that accompanied the economic downturn.

The sizable leap in emissions suggests that limiting rising average temperatures to less than 2 degrees Celsius (3.6 degrees Fahrenheit) -- a threshold that many scientists believe is crucial for preventing runaway and irreversible impacts of climate change -- will be an increasingly elusive goal.

As the IEA explained:

For this goal to be achieved, the long-term concentration of greenhouse gases in the atmosphere must be limited to around 450 parts per million of CO2-equivalent, only a 5 percent increase compared to an estimated 430 parts per million in 2000.

The IEA’s 2010 World Energy Outlook set out the 450 Scenario, an energy pathway consistent with achieving this goal, based on the emissions targets countries have agreed to reach by 2020. For this pathway to be achieved, global energy-related emissions in 2020 must not be greater than 32 [gigatonnes].

For all this math to work out, and for temperatures to keep below the 2-degree Celsius threshold, global energy-related emissions would have to rise less over the next decade than they did over just the last year, according to the IEA.

"Our latest estimates are another wake-up call," said Dr. Fatih Birol, a chief economist at the IEA, in a published statement. "The world has edged incredibly close to the level of emissions that should not be reached until 2020 if the 2-degree Celsius target is to be attained. Given the shrinking room for maneuver in 2020, unless bold and decisive decisions are made very soon, it will be extremely challenging to succeed in achieving this global goal."

The Copenhagen Accord reached in 2009 was the first time that countries involved in global climate talks informally agreed to a goal of limiting rising temperatures to no more than 2 degrees Celsius.

That benchmark was reiterated and placed on a timeline for review at talks in Cancun, Mexico, in December. Some nations, particularly those vulnerable to rising seas, believe even that amount of warming could result in catastrophic climatic changes, with attendant floods, food shortages and other impacts, over the next century.

On a per capita basis, most emissions continue to come from the developed world. But the fastest growth in new emissions is coming -- and will continue to come -- from furious economic expansion in the developing world, chiefly in China and India. Without some sort of global incentive structure that would encourage developing nations to forego fossil fuels as they expand their economies, there is increasing pessimism that targets like the 2-degrees Celsius benchmark will prove anything beyond symbolic.

The majority of the energy-related CO2 emissions last year -- 44 percent -- came from from coal, while 36 percent arose from from oil and 20 percent from natural gas, according to the IEA.